Color kinescopes of the masked-target dot-screen variety



July 17, 1956 A. M. MORRELL 2,755,402

COLOR xINEscoPEs oF THE MAsxED-TARGET DOT-SCREEN VARIETY Filed sept. 2a,195s www/fw .4 /6//7 .so Ufff E E MMM United States Patent O.

COLOR KINESCOPES F THE MASKED-TARGET DOT-SCREEN VARIETY Albert M.Morrell, East Petersburg, Pa., assignor to Radio `Corporation ofAmerica, a corporation of Delaware Application September 28, 1953,ySerial No. 382,620

6 Claims. (Cl. 313-70) This invention relates to improvements incolor-kinescopes of the so-called masked-target dot-screen variety. Suchcolor kinescopes are exemplified by Goldsmith 2,630,542 and Schroeder2,595,548 (Fig. 6).

Color-kinescopes of the kind with which the present invention isespecially concerned usually contain three electron-guns (i. e., one foreach of the primary colors). The guns are trained upon a bi-partscreen-unit consisting, essentially, of (i) a transparent viewing screenhaving a target-surface made up of a multiplicity of groups of different(e. g., red, blue and green) dot-like color-emissive areas and (ii) aforaminous electrode or shadow-mask mounted adjacent to the targetsurface of the screen. Electrons from the three guns approach thescreen-unit along a plurality of angularly related paths which, ideally,converge in the plane of the mask and pass'through the mask-apertures inthe form of miniature beams or jets which strike the dots of theparticular color allotted to each gun.

The color-dots on the screen-plateof color-tubes of the subject varietyare usually contiguous onel another. Hence, if the diameter of anlelectron jet at the plane of the screen is the same as the diameter ofa color-dot (as it must be if maximum light-output `is to be achieved),the jet will illuminate more than one color unless it impinges squarelyon the centerV of the particular dot toward which it wasdirected.

- An electron-jet will impinge squarely on the right dot only .if thevirtual area of origin of the beam from which that jet was vderived isat a particular iixed pointor color center 'in the tubesplane-of-deection. Usually, however, there is no such xed point. That isto say, during the scanning movements the color-centers of the beamsmove '76",'more or less, along the central axis of the tube, asdetermined by the maximum angle through which the beams are deectedduring said scanning movements. Furthermore, when, as is usually thecase, the kinescope is provided with one or more auxiliary coils ormagnets for dynamically converging the three beams throughout thescanning movement, the color-centers are shifted in offaxis directions.Stray magnetic-tields may also cause undesired movement of thecolor-centers. Such axial and offaxis movements of the color-centerscause the electronjets vto strike the color-dots ott-center and toinfringe upon the adjacent dots of other color-response characteristics.As a consequence, color-purity is adversely aifected, especially nearthe` edges of the screen Where the beams approach the limit of theirscanning movement.

Accordingly, the principal object of the vpresent inventionis to providean effective yet simple and inexpensive method of and means forminimizing color-dilution resulting from shifting of the color-centersof the electronbeams during their scanning movement in color-tubes ofthe masked-target dot-screen variety. u

' The foregoing and related objects are achieved in accordance withthe'preferred embodiment of the present invention by decreasing therelative diameter of the electron-jets, with respect to the, diameter ofthe individual rice v2 color-areas, as a function of the instantaneousdistance of the electron-beams from the center of the mask during thebeams scanning movement. Thus, as the beams approach the boundaries ofthe screen, the electron-jets,rde rivedV from said beams, graduallydecrease in size and, as consequence, may strike the dot-like colorareas off-center without infringing upon the next adjacent color-area orareas.

The invention is described in greater detail in connection with theaccompanying single sheet of drawings wherein:

. Fig. l is a partly diagrammatic longitudinal sectional View of a 3-guntri-color kinescope of the shadow-mask dot-screen variety, the drawingbeing marked with lines indicative of the shift in the color-centers ofits three dynamically converged beams as ythey approach their maximumangle of deflection;

`Fig. 2 is a fragmentary rear elevational view of the screen-unit ofthel color-kinescope of Fig. 1, showing a variation in size of themask-holes, as dictated by the present invention;

Pig. 3 is a chart showing a preferred correlation between mask-holediameter and radial distances of said holes from the center of the mask;

Fig. 4 is a fragmentary side elevational view of a lightbox including avariable-density lilter and a photographic plate which may be used inplotting the size and relative locations of the mask-apertures of Fig.2; and A Fig. 5 is a' rear elevation of a color-screen unit showing analternative embodiment of the invention.

The color-kinescope shown in Fig. l comprises an evacuated glass ormetal envelope 3 having a main chamber 5 which terminates at its largeend in a window 7 through which the obverse face of theglass viewingscreen 9 of a bi-part screen unit 9 11 .is visible. As shown moreclearly in Fig. 2, the viewing screen 9 is of the wellknown dot-screenvariety. As in Fig. 6 of Schroeder 2,595,548, it is provided on its rearor target surface with a multiplicity (e. g., 300,000 or more) of groupsof red (R), blue (B) and green (G) phosphor dots arranged in a hexagonalpattern, that is to say, each dot is surrounded by six other dots,alternate ones of said other dots being of a second color and theintermediate ones of said other dots being of a third color. The dotsare preferably, but not necessarily, all of the same size and may belaid down on the screen-plate by any suitable method, such, for example, as the silk (or metal) screen method described in Law-2,625,734.

The other element or shadow mask of the screen-unit comprises a thinmetal plate l1 containing a multiplicity of apertures arranged in thesame (hexagonal) pattern as the ray-sensitive screen-areas; there beingone mask-aperture for each group (of three) dot-like screen-areas. Inaccordance with the present invention and as described later on in thisspectication, the apertures or holes H, Hl, H2,getc. yin the mask arenot of a uniform size but, as shown in Fig. 2, are of substantiallyuniformly decreasing sizes as measured outwardly from the center of thearray.

The other or small end of the envelope 3 terminatesin a tubular glassneck 15 which contains a battery of three electron-guns 17, 19 and 21each of which is allotted to a particular screen color. The guns arearranged delta (A) fashion yas disclosed in Schroeder 2,595,548 and, inthe instant case, are shown provided with internal pole pieces 23, 25for maintaining the beams converged in the plane of the mask 11throughout the scanning movement, as in applicants copendingapplication, Serial No. 364,041, now U. S. Patent 2,752,520, patentedJune 26, 1956. Here, as in the Schroeder patent the required horizontaland vertical scanning movements are applied to all three of the electronbeams r, b and g by a common deecting yoke 27 which will be understoodto comprise two pairs of electromagnetic coils (indicated by the doublecurrentleads 29-31) disposed at right angles to each other on the neck15. As indicated by the single vertical line 33 thenormal"'plane-of-defiection (or center-of-scan) for the three beams r(red), b (blue) and g (green) extends through the scanning yoke 27.

The plane-of-deection or center-of-scan, mentioned in the precedingparagraph, may be defined as the plane or virtual plane in which thethree color-centers` are located, i. e. the points in which the axis ofeach deflected beam, when extended rearwardly, intersects the axis oforigin of that beam. When the three beams are undetl'ccted, i. e., whenthey are directed to the center of the target, the normalplane-of-defiection usually crosses the central axis of the tube at ornear the center of the yoke 27, as indicated by the line 33, and thecolor-l centers of the several undeflected beams lie in said plane asindicated by the points P.

The fact that the plane-of-deflection, and hence the color centers ofthe beams, are not fixed but gradually shift their positions as thebeams depart from the center of the screen unit, is illustrated in Fig.1 wherein the red" beam r is shown in solid lines at one limit of itsscanning movement. Here it will be observed, the path of the beam curvesoutwardly as it leaves the yoke 27 and, thereafter, moves in a straightline to the screen unit. If this straight portion of the red beam-path ris projected rearwardly, as indicated by the broken linesegment, it willintersect the axis of origin of the red beam at a new color center Plwhich is removed JAG" or so forwardly of its original color-center P.The shifting of the color-centers is especially complicated when, as inFig. l, udynamic convergence is employed. For example, where thekinescope is provided with internal pole pieces 23-25 (as in applicantscopending application Serial No. 364,041, now U. S. P. 2,752,520), orwith an auxiliary electromagnetic coil, or electrostatic lens system notshown, for maintaining the three beams converged in the plane of themask throughout their scanning movement. The color-centers ordinarilyshift in a direction normal to the above described forward shift, sothat the actual color-center is at some radially displaced forwardlocation such as the point P2. Asy previously mentioned, it is theconstant shifting of the color-centers during the scanning movement thatcauses the electron-jets in the mask-to-screen space to strike thephosphor-dots (R, B and G) off-center.

In applying the invention to a three-gun shadow mask tube employingdynamically converged beams, wherein the diameter of the screen wasapproximately 16 inches,

and wherein the target surface of the screen contained 342,000 groups ofred, blue and green phosphor dots (i. e., a total of 1,026,000 dots),each approximately .0136 of an inch in diameter, it was found thatcolor-dilution resulting from shifting of the color centers of the threeelectron-beams was substantially eliminated by the use of a graded-holeshadow-mask wherein the holes were approximately .008 of an inch indiameter adjacent to the center of the mask and gradually decreased indiameter to approximately .006 of an inch at the edges, or limit ofscan, as indicated by the curve in the chart of Fig. 3. As in aconventional shadow mask tube, the jets expand slightly in their transitto the screen. Thus, a jet formed by a an .008 hole near the center ofthe mask covered an area of about .011 of a .0136" phosphor dot. Havingregard for manufacturing tolerances, this is about the optimum coveragefor maximum light output. However, since the jets Vformed by the gradedmask-holes gradually decrease in diameter as the beams approach thelimits of their scanning movements, they covered less and less of thetotal area of each dot. As a consequence, although the jet wobbled (as aresult of the shift in the color center of the beam from which that jetwas derived) and struck the color-dots off-center, it did so withoutoverlapping the periphery of that dot or impinging upon the adjacentdots. The decreased light output at the edges of the screen caused bythe smaller holes was scarcely noticeable since the decrease was gradualand was less than two-to-one compared with the center of the screen.

There are several waysl of making a graded-hole shadow-mask and thepresent invention is not especially concernecl with the particulartechnique selected for its manufacture. However, in the interest ofcompleteness, one such technique is illustrated in Fig. 4. Thistechnique or method involves the use of a photographic plate 35, ashadow-mask 37 having apertures h of uniform size and a so-called gradedlight-filter 39. The uniform apertures in this mask 37 must of coursehave the same (hexagonal) pattern of distribution as that desired in thegraded mask and may be formed, for example, in the manner disclosed inLaw 2,625,734. Graded light-filters are well known in the photographicVart and may be made either (a) by photographing a white back-groundwhich is illuminated in such a way (e. g., as by a spot-light) as toprovide high brightness at the center of the photograph and decreasingbrightness toward its edges or (b) by exposing a photographic plate orfilm close to a point source of light and then photographicallyreversing the exposed film. When the graded filter 39, the uniformlyapertured mask 37 and the photographic plate 35 are exposed to a uniformlight source 41, as in Fig. 4, more light will pass through the centerof the filter 39 and mask 37 than through their intermediate and edgeportions. Hence in the Aresulting dot-like photographic pattern on theplate 35 the dots 35a are of decreasing diameter in the direction of theedges of the plate. graded-dot pattern is then reproduced,photographically, on the blank metal sheet (not shown) from which theshadow-mask is to be formed and is subjected to an acid bath whichprovides the sheet with a graded pattern of apertures corresponding tothe graded dot-like photographic pattern 35a on the plate 35. Instead ofusing a conventional photographic plate in the plotting operation andsubsequently transferring the dot-like photograph thereon to themask-blank, the blank itself may be coated with a photographic emulsionand exposed directly to the light rays passing through the mask.

Referring now to Fig. 5: The objects of the invention can also beachieved by the provision of a screenunit 41-43 wherein the holes h inthe shadow-mask 41 are of uniform (instead of non-uniform) diameter andthe dots R, B and G on the screen-plate 43 are of nonuniform (instead ofuniform) diameter. However, since the center of each color-dot in eachgroup must remain a given distance from the center c of thecorresponding mask aperture, any reduction in sim of the coloHiots canonly be achieved by increasing the separation of the dots. Separatingthe color-dots exposes the light-reecting electron-pervious metal (e.g., aluminum) film 45 on the target surface of the screen. Hence thelightreflected from the exposed portions of said film may distract theattention of the observer.

From the foregoing description it should now be ap-` parent that thepresent invention provides an eective yet simple and inexpensive methodof and means for minimizing color-dilution resulting from shifting ofthe color-centers of the electron-beams during their scanning movementin color-tubes o f the masked-target, dotscreen variety.

What is claimed is:

1. A television screen-unit comprising a screen-plate having a dot-likepattern of ray-sensitive elements on the target surface thereof, incombination with a shadowmask mounted in spaced relationship withrespect to said target surface and containing a multiplicity ofdot-,like aperture elements arranged in a pattern which isSystematically related to the pattern of dot-like elements on saidscreen-plate, the dot-like elements comprising one of said patternsbeing of substantially uniform diameter and the dot-like elementscomprising the other of said patterns diminishing in diameter outwardlyfrom a region of maximum diameter near its center.

2. A television screen-unit comprising a screen having a target-surfacemade-up of a multiplicity of systematically arranged dot-likeray-sensitive areas of substantially uniform size, in combination with ashadow-mask disposed in spaced relationship with respect to saidtargetsurface and containing a pattern of apertures which issystematically related to that of said dot-like ray-sensitive areas, theapertures in the outer portion of said pattern of apertures beingsmaller than those near its center.

3. A screen-unit comprising a screen having a target surface made up ofa multiplicity of dot-like ray-sensitive areas of substantially uniformdiameter, in combination with a shadow-mask disposed in registeredrelationship with said target surface and containing an array of dotlikeapertures of different sizes decreasing in diameter outwardly from thecenter of said array.

4. In a color-kinescope of the kind wherein a plurality ofelectron-beams are subjected to a scanning movement and pass alongdiscrete angularly related paths through the apertures of a foraminousmask in the form of electron-jets of reduced diameter in their transitto respectively different dot-like color-areas on a nearby screen, theimprovement which comprises: means for altering the relative diameter ofsaid electron-jets with respect to the diameter of said dot-likecolor-areas as a function of the instantaneous distance of saidelectron-beams from the center of said mask during said scanningmovement.

5. In a multiple-beam color-kinescope wherein the electron-beams scan ascreen-unit of the kind comprising a mask containing a multiplicity ofdot-like apertures through which beam-electrons normally pass in theform of electron-jets of reduced diameter in their transit topre-selected dot-like color-areas on the target-surface of a nearbyscreen, the improvement which comprises: means for decreasing therelative diameter of said electron-jets with respect to the diameter ofsaid color-areas as a function of the instantaneous distance of saidelectron-beams from the center of said mask.

6. In a multiple-beam color-kinescope wherein the electron-beams aresubjected to dynamic convergence during the scanning of a screen-unit ofthe kind comprising a mask containing a multiplicity of dot-likeapertures through which beam-electrons normally pass in the form ofdiscrete electron-jets of reduced diameter in their transit topre-selected dot-like color-areas of substantially uniform diameter onthe target-surface of a nearby screen, the improvement which comprises:means for decreasing the relative diameter of said electron-jets withrespect to the uniform diameter of said color-areas as a function of theinstantaneous distance of said dynamically converged electron-beams fromthe center of said mask.

References Cited in the tile of this patent UNITED STATES PATENTS2,611,099 Jenny Sept. 16, 1952 2,663,821 Law Dec. 22, 1953 2,675,501Friend Apr. 13, 1954 2,687,493 Kirkwood Aug. 24, 1954 2,716,718Sonnenfeldt Aug. 30, 1955

